Theorem: $\operatorname{Gal}(\mathbb{Q}(\sqrt[3] 2,\zeta_3)/\mathbb{Q}) =\langle\sigma, \tau\rangle\cong S_3$
My question: Why is $\sigma\tau\neq\tau\sigma$ in $\operatorname{Gal}(\mathbb{Q}(\sqrt[3] 2,\zeta_3)/\mathbb{Q})?$
My attempt: I take $\sigma(\sqrt[3] 2, \zeta_3)=( \sqrt[3]2\zeta_3, \zeta_3)$ and $\tau(\sqrt[3] 2, \zeta_3)=(\sqrt[3] 2,\zeta_3^2)$
Then $\sigma\tau(\sqrt[3] 2, \zeta_3)=\sigma (\sqrt[3]2,\zeta_3^2)=( \sqrt[3]2\zeta_3, \zeta_3^2)\tag1$
similarly $ \tau \sigma(\sqrt[3] 2, \zeta_3) =\tau( \sqrt[3]2\zeta_3, \zeta_3)=( \sqrt[3]2\zeta_3, \zeta_3^2)\tag2$
From $(1)$ and $(2) $
we have $$\sigma\tau=\tau\sigma$$
This implies $\operatorname{Gal}(\mathbb{Q}(\sqrt[3] 2,\zeta_3)/\mathbb{Q}$ is not isomorphic to $S_3$
Tell me where I'm wrong?
Best Answer
Well,
$$ \sigma\tau(\sqrt[3]2,\zeta_3)=\sigma(\sqrt[3]2,\zeta_3^2)=(\sqrt[3]2\zeta_3^\color{red}1,\zeta_3^2) $$
while
$$ \tau\sigma(\sqrt[3]2,\zeta_3)=\tau(\sqrt[3]2\zeta_3,\zeta_3)=(\sqrt[3]2\zeta_3^\color{red}2,\zeta_3^2)\,. $$